(CNN) — At times with only flashlights to illuminate the way, NYU Langone Medical Center was evacuating some 260 patients, carrying some of them down 15 flights of stairs to awaiting ambulances ready to take them to the safety of other hospitals.

NYU didn’t anticipate such heavy flooding from Sandy, the superstorm that hit Monday, and chose not to evacuate all its patients before the storm, as they did with Hurricane Irene a year ago. But between 7 and 7:45 p.m. Monday, the hospital’s basement, lower floors, and elevator shafts filled with 10 to 12 feet of water, and the hospital lost its power, according to Dr. Andrew Brotman, senior vice president and vice dean for clinical affairs and strategy.

“Things went downhill very, very rapidly and very unexpectedly,” he said. “The flooding was just unprecedented.”

Emergency generators did kick in, but two hours later, about 90% of that power went out, and the hospital decided to evacuate.

By 1:30 a.m., about half the patients had been evacuated, including all the patients in the adult, pediatric, and newborn intensive care units. Brotman said he anticipated the evacuation would last until around 6:30 a.m.

Four of the newborns were on respirators that were breathing for them, and when the power went out, each baby was carried down nine flights of stairs while a nurse manually squeezed a bag to deliver air to the baby’s lungs.

The adult respirators have batteries, so those patients have not needed manual respiration, he added.

Some 1,000 staff members — doctors, nurses, residents, and medical students — along with firefighters and police officers are evacuating the patients. Since about 10% of the backup power was working, there were a few lights on in the hallways. But still much of the work was being done by flashlight.

“Everybody’s digging in and doing what they have to do,” Brotman said.

The hospital usually has about 800 patients, but discharged hundreds of patients over the weekend in anticipation of the storm, he said.

But no one anticipated the high flood levels, or that the generators, which are on top of the hospital, would get waterlogged.

During Irene, only one building was flooded, and with just two to three feet of water. Monday night, seven hospital buildings were flooded with 10 to 12 feet of water each, including the medical school and the Smilow Research Center, which was built about three years ago.

“It had a very sophisticated foundation that was built specifically to withstand a flood, but it flooded anyway,” he said. “That’s just an example of how stunning and rapid this flooding situation was.”